Light Emitting Device and Method of Manufacturing the Same

ABSTRACT

It is an object of the present invention to provide a high-contrast light-emitting device without using a polarization plate. In particular, it is an object of the present invention to make contrast control simpler for a light-emitting device provided with a color filter. 
     A light-emitting device according to the present invention has a feature of having a structure for reducing reflection of light from a light-emitting later at a reflective electrode, and further, has a feature of absorbing wavelengths other than the light by a color filter to enhance the contrast. Accordingly, contrast control can be performed in consideration of only a luminescence component from the light-emitting layer, and is thus made simpler.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-emitting device usingluminescence such as electroluminescence, and more particularly relatesto a light-emitting device with a light-emitting element that is formedby interposing a light-emitting layer between a pair of electrodes.

2. Description of the Related Art

A display device including a light-emitting element (hereinafter,referred to as a light-emitting device) has advantages such as wideviewing angle, low power consumption, and rapid response speed ascompared with a liquid crystal display device, and research anddevelopment thereof have been actively carried out.

-   -   The light-emitting element has a structure of a luminescent        material provided between a pair of electrodes, and light from        the luminescent material is extracted depending on the        light-transmitting properties of the electrodes.

For example, in the case of desiring to extract light toward onedirection, a light-transmitting material is used for one electrodeprovided in the direction where light is extracted whereas the otherelectrode is formed by using a material that has no light-transmittingproperty, that is, a reflective material. The light extractionefficiency can be enhanced by using reflection at the other electrodeeffectively.

At the same time, when this reflective material is used for theelectrode, reflection of outside light becomes a problem. In order toprevent reflection of outside light, a structure provided with apolarization plate or a circular polarization plate should beconsidered. However, there is fear that light from the light-emittingelement is lost when a polarization plate or the like is used.

As a method for preventing reflection of outside light, a display deviceis proposed, where the optical distance between a pair of electrodes ismade to satisfy a given formula, a resonator structure is introduced,and the resonance wavelength coincides with the peak wavelength of aspectrum of light to be extracted (refer to Patent Document 1).

-   [Patent Document 1] Japanese Patent Application Laid-Open No.    2004-178930

SUMMARY OF THE INVENTION

Patent Document 1 describes a light-emitting element in which theoptical distance between edges on a light-emitting element side of firstand second electrodes satisfies an optical distance to improve the colorpurity by multiple interference. This optical distance is controlled byvarying the film thickness of a light-emitting element for each of red,green, blue colors. When the film thicknesses of the light-emittingelements are varied as described above, the driving voltages of therespective light-emitting element are different from each other, and inparticular, the driving voltage of the light-emitting element isincreased as the film thickness is thicker.

In addition, it is described in Patent Document 1 that the firstelectrode is formed with the use of palladium (Pt), gold (Au), silver(Ag), chromium (Cr), or tungsten (W) as a material that has the highestpossible reflectivity, and that a color filter absorbs outside lightreflected at a wiring to improve the contrast.

However, it is believed that only insufficient antireflection effectagainst outside light can be obtained when a material that has a highreflectivity is used for the first electrode and a color filter isprovided in order to absorb outside light reflected at the firstelectrode, which leads to decrease in contrast.

Consequently, it is an object of the present invention to provide ahigh-contrast light-emitting device. Specifically, it is an object ofthe present invention to provide a light-emitting device with contrastincreased without using a polarization plate.

In view of the objects described above, the present invention has afeature of a structure for reducing reflection of a specific wavelengthor specific waveband (hereinafter, referred to as a wavelength) of aluminescent color from a light-emitting element at an electrode wherelight from the light-emitting element is reflected. The structure isthat the film thickness of an electroluminescent layer between theelectrode and the opposed substrate is determined for reducingreflection of the wavelength of the luminescent color from thelight-emitting element. In this case, the film thickness of theelectroluminescent layer is determined so that reflection of thewavelength of the luminescent color from the light-emitting element isreduced at the electrode. In addition, in the case of performingfull-color display in which a wavelength of light from a light-emittingelement is different, it is a feature that a structure for reducingreflection of a wavelength of a luminescent color from eachlight-emitting element is provided.

A specific aspect of the present invention is a light-emitting deviceincluding a first electrode, a second electrode provided to be opposedto the first electrode, a light-emitting element comprising the firstelectrode and the second electrode, and a color filter through whichlight from the light-emitting element is transmitted, where refection ofa wavelength of light (referred to as an emission wavelength) emittedfrom the light-emitting element at the first electrode is reduced.Further, the light-emitting device according to the present inventionhas a feature that the color filter includes a material that has a hightransmittance with respect to the emission wavelength from thelight-emitting element. Accordingly, visible light having at least peakwavelength passes through the color filter. In addition, the presentinvention has a feature of a method for manufacturing the light-emittingdevice.

Another aspect of the present invention is a light-emitting deviceincluding a first electrode, a second electrode provided to be opposedto the first electrode, a light-emitting element comprising the firstelectrode and the second electrode, and a color filter through whichlight from the light-emitting element is transmitted, where thelight-emitting element emits monochromatic light, and refection of anemission wavelength from the monochromatic light-emitting element at thefirst electrode is reduced. Further, the light-emitting device accordingto the present invention has a feature that the color filter includes amaterial that has a high transmittance with respect to the emissionwavelength from the monochromatic light-emitting element. In addition,the present invention has a feature of a method for manufacturing thelight-emitting device.

Another aspect of the present invention is a light-emitting deviceincluding a first electrode, a second electrode provided to be opposedto the first electrode, an electroluminescent layer provided between thefirst electrode and the second electrode, a light-emitting elementcomprising the first electrode, the second electrode, and theelectroluminescent layer, and a color filter through which light fromthe light-emitting element is transmitted, where any one layer of theelectroluminescent layer has a thickness that reduces reflection of anemission wavelength from the light-emitting element at the firstelectrode. Further, the light-emitting device according to the presentinvention has a feature that the color filter includes a material thathas a high transmittance with respect to the emission wavelength fromthe light-emitting element. In addition, the present invention has afeature of a method for manufacturing the light-emitting device.

Another aspect of the present invention is a light-emitting deviceincluding a first electrode, a second electrode provided to be opposedto the first electrode, an electroluminescent layer provided between thefirst electrode and the second electrode, a light-emitting elementcomprising the first electrode, the second electrode, and theelectroluminescent layer, and a color filter through which light fromthe light-emitting element is transmitted, where the light-emittingelement emits monochromatic light, any one layer of theelectroluminescent layer has a thickness that reduces reflection of anemission wavelength from the monochromatic light-emitting element at thefirst electrode. Further, the light-emitting device according to thepresent invention has a feature that the color filter includes amaterial that has a high transmittance with respect to the emissionwavelength from the monochromatic light-emitting element. In addition,the present invention has a feature of a method for manufacturing thelight-emitting device.

Another aspect of the present invention is a light-emitting devicecomprising, a first electrode, a light emitting layer adjacent to thefirst electrode, a second electrode adjacent to the light emitting layerwith the light emitting layer interposed between the first and secondelectrodes, a color filter adjacent to the second electrode with thesecond electrode interposed between the light emitting layer and thecolor filter, where a reflectance of light emitted from said lightemitting layer at the first electrode is 10% or less.

Another aspect of the present invention is a light-emitting devicecomprising, a first electrode, a light emitting layer adjacent to thefirst electrode, a second electrode adjacent to the light emitting layerwith the light emitting layer interposed between the first and secondelectrodes, a color filter adjacent to the second electrode with thesecond electrode interposed between the light emitting layer and thecolor filter, where a reflectance of light emitted from said lightemitting layer at the first electrode is 10% or less, and where visiblelight having at least peak wavelength passes through the color filter.

Another aspect of the present invention is a light-emitting devicecomprising, a first electrode, a light emitting layer adjacent to thefirst electrode, a second electrode adjacent to the light emitting layerwith the light emitting layer interposed between the first and secondelectrodes, a color filter adjacent to the second electrode with thesecond electrode interposed between the light emitting layer and thecolor filter, where a reflectance of light emitted from said lightemitting layer at the first electrode is 10% or less, where visiblelight having at least peak wavelength passes through the color filter,and where the light emitting layer emits monochromatic light.

Another aspect of the present invention is a light-emitting devicecomprising, a first electrode, a electroluminescent layer adjacent tothe first electrode, a second electrode adjacent to the light emittinglayer with the light emitting layer interposed between the first andsecond electrodes, a color filter adjacent to the second electrode withthe second electrode interposed between the light emitting layer and thecolor filter, where the electroluminescent layer having the layerincluding a metal oxide, where the layer including a metal oxide athickness that 10% or less reflectance of an emission wavelength fromthe electroluminescent layer at the first electrode, where visible lighthaving at least peak wavelength passes through the color filter, andwhere the light-emitting element emits monochromatic light.

In the light-emitting device according to the present invention, thelayer of the electroluminescent layer, which has the thickness thatreduces reflection of the emission wavelength from the light-emittingelement, has a layer including a metal oxide selected from the groupconsisting of a vanadium oxide, a molybdenum oxide, a niobium oxide, arhenium oxide, a tungsten oxide, a ruthenium oxide, a titanium oxide, achromium oxide, a zirconium oxide, a hafnium oxide, and a tantalumoxide. In addition, the present invention has a feature of a method formanufacturing the light-emitting device.

In the present invention, the color filter may include a colorconversion layer.

In particular, the present invention has a feature of a light-emittingdevice that emits a monochromatic luminescent color. As compared with afull-color light-emitting device, this monochromatic light-emittingdevice causes no color mixture or has no viewing angle dependence evenwhen the distance between a substrate over which a transistor isprovided and an opposed substrate that is opposed to the TFT substrateis large. In addition, restriction of alignment can be alleviated whenthe TFT substrate is attached to the opposed substrate. Further, themanufacturing process is simple since it is not necessary to form colorfilters separately for each color. Moreover, the definition can be madehigher in the same design rule as that of a full-color light-emittingdevice.

In addition, antireflection coating (AR coating: Anti-Reflectiontreatment coating) may be performed to one or both sides of thesubstrate to which light is emitted, the TFT substrate or the opposedsubstrate.

Moreover, the present invention is suitable for a top-emissionlight-emitting device that has an opposed substrate to whichluminescence is emitted. Reflection of outside light can be effectivelyprevented by using a light-emitting element that has an electrodestructure according to the present invention and additionally using acolor filter for a top-emission light-emitting device. In addition, alight-emitting element that has an electrode structure according to thepresent invention can be applied also to a bottom-emissionlight-emitting device that has a TFT substrate to which luminescence isemitted. However, it is necessary to consider a protective film a gateinsulating film, and a wiring layer that are provided between theelectrode and the substrate.

According to the present invention, a light-emitting element in whichunnecessary reflection at an electrode is reduced and a light-emittingdevice that has the light-emitting element can be provided, and onlyluminescence from the light-emitting element is transmitted through acolor filter or the like included in the light-emitting device to enablecontrol of the contrast and the like.

According to the present invention described above, a light-emittingdevice that requires no polarization plate or the like can be provided.Accordingly, luminescence from a light-emitting element is notattenuated. Further, it is possible to reduce costs since the presentinvention requires no polarization plate or the like although thepolarization plate or the like is expensive. Moreover, although there isa problem that the polarization plate or the like is fragile, thisproblem is not caused either.

Further, the present invention can be applied to a monochromaticlight-emitting device, and low cost can be thus achieved as comparedwith a full-color light-emitting device.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is an enlarged view of a light-emitting element of alight-emitting device according to the present invention;

FIG. 2 is an enlarged view of a light-emitting element of alight-emitting device according to the present invention;

FIG. 3 is a cross-sectional view illustrating a light-emitting elementaccording to the present invention;

FIG. 4 is a cross-sectional view illustrating a light-emitting elementaccording to the present invention;

FIG. 5 is a cross-sectional view illustrating a light-emitting deviceaccording to the present invention;

FIG. 6 is a cross-sectional view illustrating a light-emitting deviceaccording to the present invention;

FIGS. 7A to 7D are pixel circuit diagrams for a light-emitting deviceaccording to the present invention;

FIGS. 8A to 8F are diagrams illustrating electronic devices to which alight-emitting device according to the present invention is applied.

FIG. 9 is results for an electrode reflectivity of a red light-emittingelement;

FIG. 10 is results for an electrode reflectivity of a greenlight-emitting element;

FIG. 11 is results for an electrode reflectivity of a bluelight-emitting element;

FIG. 12 is a cross-sectional view illustrating a light-emitting deviceaccording to the present invention; and

FIG. 13 is a cross-sectional view illustrating a light-emitting deviceaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiment modes of the present invention will be described below withreference to the accompanying drawings. However, the present inventionmay be embodied in a lot of different forms, and it is to be easilyunderstood that various changes and modifications will be apparent tothose skilled in the art unless such changes and modifications departfrom the scope of the present invention. Therefore, the presentinvention is not to be construed with limitation to what is described inthe embodiment modes. It is to be noted that the same reference numeraldenotes the same portion or a portion that has the same function in theall drawings for describing the embodiments, and repeated description ofthe portion will be omitted.

Embodiment Mode 1

In the present embodiment, a light-emitting element that has a structurefor lowering the reflectivity of a reflective electrode and alight-emitting device that has a color filter will be described. In thepresent embodiment mode, the light-emitting device is described as amonochromatic light-emitting device. FIG. 1 shows a light-emittingelement that has a first electrode 101, a second electrode 102 that isopposed to the first electrode 101, and between these electrodes, firstto third layers 111 to 113 provided in this order from the firstelectrode 101 side. The first electrode 101 has a light-reflectingproperty, and the second electrode 102 has a light-transmittingproperty. It is to be noted that a light-transmitting property can beobtained by forming the second electrode 102 with the use of alight-transmitting material. However, even when a material that has nolight-transmitting property is used, a light-transmitting property canbe obtained by making the second electrode 102 thinner to such a degreethat the light-transmitting property is provided. Further, a colorfilter 105 is provided in a direction of light emission, that is, on thesecond electrode 102 side. The color filter 105 can be formed by screenprinting, droplet discharge, or the like with the use of a knownmaterial. It is to be noted that the first electrode 101, the secondelectrode 102, the first to third electrodes 111 to 113 can be formed byevaporation and can be formed continuously without exposing the air.

Further, it is a feature that the reflectivity of the first electrode101 with respect to an emission wavelength emitted from any one of thefirst to third layers 111 to 113 is reduced. Specifically, the filmthickness of at least one of the first to third layers 111 to 113 isdetermined so as to reduce the reflectivity at the first electrode 101.Visible light having at least emission wavelength passes through thecolor filter. In particular, it is preferable to determine the filmthickness of the first layer 111. The film thickness of the first layer111 is determined so that a layer corresponding to a light-emittinglayer in which an electron and a hole are recombined among the first tothird layers 111 to 113. The result is that the reflectivity of thefirst electrode 101 with respect to an emission wavelength of lightemitted from any one of the first to third layers 111 to 113 can bereduced. Further, only luminescence emitted from any one of the first tothird layer 111 to 113 can be obtained without considering reflectedlight. Accordingly, the contrast and the like can be controlled inconsideration of only the luminescence, namely, control of the controland the like can be made simpler since light of a component reflected atthe first electrode 101 is not considered.

On the other hand, Patent Document 1 discloses a technical idea that afirst electrode has a higher reflectivity as much as possible, which isdifferent from the technical idea of the present invention.

Further, the color filter 105 is provided so as to increase thetransmittance of an emission wavelength from the light-emitting elementdescribed above. Accordingly, visible light having at least peakwavelength passes through the color filter. The result is that the colorfilter 105 is able to absorb light other then the luminescent color andno component of outside light enters the first electrode 101.Accordingly, the contrast can be enhanced.

Briefly, according to the present invention, the light-emitting elementthat provides specific light can act like a polarization plate byreducing reflection of the light at the first electrode 101 andsuppressing wavelengths other than the wavelength of the light with theuse of the color filter 105. In this case, light is emitted from thesecond electrode 102 opposed to the first electrode 101 (a top-emissionlight-emitting device).

In addition, in the present invention, since the film thickness any oneof the first to third layers 111 to 113 is varied, it is necessary tomake a specific layer thicker. Consequently, it is a feature that amixed layer including an organic compound and a metal oxide that is aninorganic compound (referred to as a layer including a metal oxide) isused for the layer that is made thicker. It is to be noted that themixed layer includes a layer in which the organic compound and theinorganic compound are mixed and a layer in which the organic compoundand the inorganic compound are laminated.

In general, it is not preferable to make a layer of a light-emittinglayer thicker since the driving voltage is increased. However, when themixed layer including the organic compound and the metal oxide is used,the driving voltage itself can be made lower, and further, the drivingvoltage is not increased even when the mixed layer is made thicker. Theuse of this mixed layer including the organic compound and the inorganiccompound makes it possible to suppress crystallization of the organiccompound and make the mixed layer thicker without increase inresistance. Therefore, even when there is irregularity due to dust,contamination, and the like on a substrate, the irregularity has almostno influence. Accordingly, defects such as a short circuit between thefirst electrode 101 and the second electrode 102 due to irregularity canbe prevented, with the result that the mass productivity can beenhanced.

In addition, a full-color light-emitting device may be formed although amonochromatic light-emitting device is described in the presentembodiment mode. In the case of a full-color light-emitting device, forexample, as shown in FIG. 12, color filters 105R, 105G, and 105B may beprovided for regions from which a red (R), a green (G), and a blue (B)are produced, respectively. Further, the film thickness of any one offirst to third layers 111 to 113, preferably the film thickness of thefirst layer 111, is determined so that a first electrode 101 reflectslight of each color of RGB at a lower reflectivity.

As described above, reflection of light from the light-emitting elementat the first electrode 101 can be reduced, and the color filter 105makes it possible to prevent wavelengths other than a luminescent colorfrom the light-emitting element from being transmitted. Accordingly, thecontrast can be enhanced by adjusting only light from the light-emittingelement, and a light-emitting device that requires no polarization plateor the like can be provided.

Embodiment Mode 2

In the present embodiment mode, FIG. 2 shows a case where a color filter105 is provided is provided on the first electrode 101 side unlike FIG.1, and light is emitted from the first electrode 101 (a bottom-emissionlight-emitting device).

In the case of providing the color filter 105 on the first electrode 101side in this way, a plurality of insulating films constituting a thinfilm transistor and the like, which are stacked, are provided below thefirst electrode 101 (corresponding to between the first electrode 101and a substrate). Therefore, it is preferable to determine the filmthicknesses of the first to third layers 111 to 113 also inconsideration of light that is reflected between these insulating filmsand the like. In addition, in a region through which light istransmitted, the insulating films and the like may be removed

As described above, the present invention has a feature that the filmthickness of the any one of the first to third layers 111 to 113 isdetermined so that the reflectivity of the second electrode 102 withrespect to an emission wavelength of emitted light is reduced. In thiscase, it is preferable to determine the film thickness of the thirdlayer 113 between a light-emitting layer in which an electron and a holeare recombined and the second electrode 102 among the first to thirdlayers 111 to 113. Further, it is preferable to use a mixed layerincluding an organic compound and an inorganic compound for a layer thatis required to be made thicker since the driving voltage is notincreased.

In addition, a full-color light-emitting device may be formed although amonochromatic light-emitting device is described in the presentembodiment mode. In the case of a full-color light-emitting device, forexample, as shown in FIG. 13, color filters 105R, 105G, and 105B may beprovided for regions from which a red (R), a green (G), and a blue (B)are produced, respectively. Further, the film thickness of any one offirst to third layers 111 to 113, preferably the film thickness of thethird layer 113, is determined so that a second electrode 102 reflectslight of each color of RGB at a lower reflectivity.

As described above, reflection of light from the light-emitting elementat the second electrode 102 can be reduced, and the color filter 105makes it possible to prevent wavelengths other than a luminescent colorfrom the light-emitting element from being transmitted. Accordingly, thecontrast can be enhanced by adjusting only light from the light-emittingelement, and a light-emitting device that requires no polarization plateor the like can be provided.

Embodiment Mode 3

In the present embodiment mode, the structure of a light-emittingelement will be described.

As shown in FIG. 3, the light-emitting element according to the presentinvention has a first electrode 101 and a second electrode 102 that areopposed to each other, and has a first layer 111, a second layer 112,and a third layer 113 that are stacked in this order from the firstelectrode 101 side. When a voltage is applied to this light-emittingelement, for example, so that the potential of the first electrode 101is higher than the potential of the second electrode 102, a hole isinjected from the first layer 111 into the second layer 112, and anelectron is injected from the third layer 113 into the second layer 112.Then, the hole and the electron are recombined to excite a luminescentmaterial, and luminescence is produced when the excited luminescentmaterial returns to the ground state.

Next, the first to third layers 111 to 113, the first electrode 101, andthe second electrode 102 will be described.

The first layer 111 is a layer that generates holes. This function canbe achieved by using, for example, a layer including a hole transportingmaterial and a material that exhibits an electron accepting property tothe hole transporting material. In addition, it is preferable that thematerial that exhibits an electron accepting property to the holetransporting material be included so that the molar ratio of thematerial to the hole transporting material is 0.5 to 2 (=the materialthat exhibits an electron accepting property to the hole transportingmaterial/the hole transporting material).

The hole transporting material is a material in which electrons aretransported more then electrodes, and for example, organic compounds,aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: α-NPD),4,4′-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (abbreviation: TPD),4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]-triphenylamine(abbreviation: MTDATA), 4,4′-bis[N-{4-(N,N-di-m-tolylamino)phenyl}-N-phenylamino]biphenyl (abbreviation: DNTPD), and phthalocyaninecompounds such as phthalocyanine (abbreviation: H₂Pc) and copperphthalocyanine (abbreviation: CuPc), can be used as the holetransporting material. It is to be noted that the hole transportingmaterial is not to be considered limited to these.

In addition, an oxide of a transition metal belonging to any one ofGroup 4 to 12 of the periodic table (a metal oxide) can be used as thematerial that exhibits an electron accepting property to the holetransporting material. Among others, an oxide of a transition metalbelonging to any one of Groups 4 to 8 of the periodic table often has ahigher electron accepting property, and a vanadium oxide, a molybdenumoxide, a niobium oxide, a rhenium oxide, a tungsten oxide, a rutheniumoxide, a titanium oxide, a chromium oxide, a zirconium oxide, a hafniumoxide, and a tantalum oxide are particularly preferable. Besides theoxides, nitrides and oxynitrides of the metals mentioned above may beused. It is to be noted that the material that exhibits am electronaccepting property to the hole transporting material is not to beconsidered limited to these.

Further, it is preferable to form the first layer 111 by using a layerin which the hole transporting material, which is composed of an organicmaterial, and the material that exhibits an electron accepting propertyto the hole transporting material, which is composed of the metal oxidementioned above, are mixed. Crystallization of the organic compound thatis used for the first layer 111 can be suppressed by using this layer inwhich the organic material and the inorganic material are mixed, and thefirst layer 111 can be thus formed to be thicker without increase inresistance. Therefore, even when there is irregularity due to dust,contamination, and the like on a substrate, the irregularity has almostno influence since the first layer 111 is made thicker. Accordingly,defects such as a short circuit between the first electrode 101 and thesecond electrode 102 due to irregularity can be prevented. Further, thelayer in which the material that exhibits an electron accepting propertyto the hole transporting material, which is composed of the metal oxidementioned above, is mixed has a higher conductivity, and thus, can bemade thicker. Since the first electrode 101 and the second layer 112 canbe kept further away from each other by making the first layer 111thicker in this way, quenching of luminescence due to a metal can beprevented.

Further, the first layer 111 may include another organic compound. Asthe organic compound, rubrene and the like can be cited. The reliabilitycan be improved by the addition of rubrene.

In addition to this, the first layer 111 may be a layer composed of ametal oxide such as a molybdenum oxide, a vanadium oxide, a rutheniumoxide, a cobalt oxide, and a copper oxide. Alternatively, other than themetal oxide, the first layer 111 may be a layer composed of a metalnitride including this metal element.

This first layer 111 can be formed by evaporation. When a layerincluding a plurality of mixed compounds is used as the first layer 111,co-evaporation can be used. The co-evaporation includes co-evaporationby resistance-heating evaporation, co-evaporation by electron-beamevaporation, and co-evaporation by resistance-heating evaporation andelectron-beam evaporation, and in addition, there are methods such asdeposition by resistance-heating evaporation and sputtering anddeposition by electron-beam evaporation and sputtering. The first layer111 can be formed by combining the same type of methods or differenttypes of methods. In addition, the example described above shows a layerincluding two kinds of materials. However, when three or more kinds ofmaterials are included, the first layer 111 can be formed also in thesame way by combining the same type of methods or different types ofmethods.

Next, the second layer 112 that is a layer including a light-emittinglayer will be described. The layer including the light-emitting layermay be a single layer composed of only the light-emitting layer or amultilayer. To cite a case, a specific multilayer includes alight-emitting layer and additionally a plurality of layers selectedfrom electron transporting layers and hole transporting layers. In FIG.3, a multilayer case in which the second layer 112 includes a holetransporting layer 122, a light-emitting layer 123, and an electrontransporting layer 124 is shown.

The hole transporting layer 124 can be formed with the use of a knownmaterial. Typical examples include aromatic amine compounds, forexample, aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (hereinafter, referredto as α-NPD), 4,4′,4″-tris(N,N-diphenyl-amino)triphenylamine(hereinafter, referred to as TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenyl-amino]-triphenylamine(hereinafter, referred to as MTDATA).

It is preferable that the light-emitting layer 123 be a layer includinga luminescent material dispersed in a material that has a larger energygap than the luminescent material. It is to be noted that the energy gapindicates the energy gap between the LUMO level and the HOMO level. Inaddition, a material that provides a favorable luminous efficiency andis capable of producing luminescence of a desired emission wavelengthmay be used for the luminescent material.

For the material that is used for dispersing the luminescent material,for example, anthracene derivatives such as anthracene derivatives suchas 9,10-di(2-naphthyl)-2-tert-butylanthracene (abbreviation: t-BuDNA),carbazole derivatives such as 4,4′-bis(N-carbazolyl)-biphenyl(abbreviation: CBP), and metal complexes such asbis[2-(2-hydroxyphenyl)-pyridinato]zinc (abbreviation: Znpp₂) andbis[2-(2-hydroxyphenyl)-benzoxazolato]zinc (abbreviation: ZnBOX) can beused. However, the material that is used for dispersing the luminescentmaterial is not limited to these materials. When the luminescentmaterial is dispersed, concentration quenching of luminescence from theluminescent material can be prevented.

Next, luminescent materials for the light-emitting layer 123 will bementioned. When red luminescence is desired to be obtained,4-dicyanomethylene-2-isopropyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran(abbreviation: DCJTI),4-dicyanomethylene-2-methyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran(abbreviation: DCJT),4-dicyanomethylene-2-tert-butyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran(abbreviation: DCJTB), periflanthene, and2,5-dicyano-1,4-bis-[2-(10-methoxy-1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-benzene,bis[2,3-bis(4-fluorophenyl) quinoxalinato]iridium (acetylacetonato)(abbreviation: Ir(Fdpq)₂(acac)), and the like can be used. However, thematerial for obtaining red or reddish luminescence is not limited tothese materials, and a material that produces luminescence with a peakwavelength from 600 nm to 680 nm in an emission spectrum can be used.

When green luminescence is desired to be obtained,N,N′-dimethylquinacridone (abbreviation: DMQd), coumarin 6, coumarin545T, tris(8-quinolinolato) aluminum (abbreviation: Alq₃), and the likecan be used. However, the material for obtaining green or greenishluminescence is not limited to these materials, and a material thatproduces luminescence with a peak wavelength from 500 nm to 550 nm in anemission spectrum can be used.

In addition, when blue luminescence is desired to be obtained,9,10-di(2-naphthyl)-tert-butylanthracene (abbreviation: t-BuDNA),9,9′-bianthryl, 9,10-diphenylanthracene (abbreviation: DPA),9,10-bis(2-naphthyl)anthracene (abbreviation: DNA),bis(2-methyl-8-quinolinolato)-4-phenylphenolato-gallium (abbreviation:BGaq), bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum(abbreviation: BAlq), and the like can be used for the light-emittinglayer 123. However, the material for obtaining blue or bluishluminescence is not limited to these materials, and a material thatproduces luminescence with a peak wavelength from 420 nm to 500 nm in anemission spectrum can be used.

In the case of forming monochromatic light-emitting device, theselection is possible from the luminescent materials for the threecolors, and further, a color filter makes it possible to produce desiredlight emission.

A layer in which a metal oxide is mixed in this organic compound thatserves as the light-emitting layer may be used.

Next, the electron transporting layer 124 will be described. Theelectron transporting layer 124 is a layer that has a function oftransporting electrons injected from the second electrode 102 to thelight-emitting layer 123. By providing the electron transporting layer124 in this way to keep the second electrode 102 and the light-emittinglayer 123 further away from each other, quenching of luminescence due toa metal can be prevented.

It is preferable that the electron transporting layer 124 be formed withthe use of a material in which the hole mobility is higher than theelectron mobility. Further, it is more preferable that the electrontransporting layer 124 be formed with the use of a material that has anelectron mobility of 10⁻⁶ cm²/Vs or more. In addition, the electrontransporting layer 124 may be a layer that has a multilayer structureformed by combining two or more layers each including the materialdescribed above. As a specific material for the electron transportinglayer 124, a metal complex having a quinoxaline skeleton or abenzoquinoline skeleton, such as tris(8-quinolinolato) aluminum(abbreviation: Alq₃), tris(4-methyl-8-quinolinolato) aluminum(abbreviation: Almq₃), bis(10-hydroxybenzo[h]-quinolinato) beryllium(abbreviation: BeBq₂), and BAlq mentioned above, is preferred. Inaddition, a metal complex having an oxazole-based or thiazole-basedligand, such as bis[2-(2-hydroxyphenyl)-benzoxazolato]zinc(abbreviation: Zn(BOX)₂) andbis[2-(2-hydroxyphenyl)-benzothiazolato]zinc (abbreviation: Zn(BTZ)₂),can be used. Moreover, besides metal complexes,2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene(abbreviation: OXD-7),3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: TAZ),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen),bathocuproin (abbreviation: BCP), and the like can be also used.

This second layer 112 can be manufactured by evaporation. When a mixedlayer is formed for the second layer 112, co-evaporation can be used.The co-evaporation includes co-evaporation by resistance-heatingevaporation, co-evaporation by electron-beam evaporation, andco-evaporation by resistance-heating evaporation and electron-beamevaporation, and in addition, there are methods such as deposition byresistance-heating evaporation and sputtering and deposition byelectron-beam evaporation and sputtering. The first layer 111 can beformed by combining the same type of methods or different types ofmethods. In addition, the example described above shows a layerincluding two kinds of materials. However, when three or more kinds ofmaterials are included, the first layer 111 can be formed also in thesame way by combining the same type of methods or different types ofmethods as described above.

Next, the third layer 113 that is layer that generates electrons will bedescribed. As this third layer 113, for example, a layer including anelectron transporting material and a material that exhibits an electrondonating property to the electron transporting material can be cited.

It is to be noted that the electron transporting material is a materialin which more electrons are transported than holes, and for example,metal complexes such as tris(8-quinolinolato) aluminum (abbreviation:Alq₃), tris(4-methyl-8-quinolinolato) aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]-quinolinato)beryllium (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviation:BAlq), bis[2-(2-hydroxyphenyl)-benzoxazolato]zinc (abbreviation:Zn(BOX)₂), and bis[2-(2-hydroxyphenyl)-benzothiazolato]zinc(abbreviation: Zn(BTZ)₂), and further,2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene(abbreviation: OXD-7),3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: TAZ),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen),bathocuproin (abbreviation: BCP), and4,4-bis(5-methyl-benzoxazol-2-yl)stilbene (abbreviation: BzOs) can beused for the electron transporting material. In addition, the thirdlayer 113 can be formed with the use of an n-type semiconductor.However, the electron transporting material is not limited to these.

In addition, for the material that exhibits an electron donatingproperty to the electron transporting material, a substance selectedfrom alkali metals and alkali-earth metals, specifically such as lithium(Li), calcium (Ca), sodium (Na), potassium (K), and magnesium (Mg), canbe used. Further, specific materials include oxides of the alkalimetals, oxides of the alkali-earth metals, nitrides of the alkalimetals, and nitrides of the alkali-earth metals, specifically, a lithiumoxide (Li₂O), a calcium oxide (CaO), a sodium oxide (Na₂O), a potassiumoxide (K₂O), and a magnesium oxide (MgO), lithium fluoride (LiF), cesiumfluoride (CsF), and calcium fluoride (CaF₂). However, the material thatexhibits an electron donating property to the electron transportingmaterial is not limited to these. It is to be noted that it ispreferable that the material that exhibits an electron donating propertyto the electron transporting material be included so that the molarratio of material that exhibits an electron donating property to theelectron transporting material to the electron transporting material is0.5 to 2 (=the material that exhibits an electron donating property tothe electron transporting material/the electron transporting material).

Alternatively, the third layer 113 may be a layer composed of a materialsuch as zinc oxide, zinc sulfide, zinc selenide, tin oxide, or titaniumoxide.

Further, it is preferable to form the third layer 113 by using a layerin which the electron transporting material, which is composed of anorganic material, and the material that exhibits an electron donatingproperty to the electron transporting material, which is composed of themetal oxide mentioned above, are mixed. Crystallization of the organiccompound that is used for the third layer 113 can be suppressed by usingthis layer in which the organic material and the inorganic material aremixed, and the third layer 113 can be thus formed to be thicker withoutincrease in resistance. Therefore, even when there is irregularity dueto dust, contamination, and the like on a substrate, the irregularityhas almost no influence since the third layer 113 is made thicker.Accordingly, defects such as a short circuit between the first electrode101 and the second electrode 102 due to irregularity can be prevented.Further, the layer in which the material that exhibits an electrondonating property to the hole electron transporting material, which iscomposed of the metal oxide mentioned above, is mixed has a higherconductivity, and thus, can be made thicker. Since the first electrode101 and the second layer 112 can be kept further away from each other bymaking the third layer 113 thicker in this way, quenching ofluminescence due to a metal can be prevented.

This third layer 113 can be manufactured by evaporation. When a mixedlayer is formed for the third layer 113, co-evaporation can be used. Theco-evaporation includes co-evaporation by resistance-heatingevaporation, co-evaporation by electron-beam evaporation, andco-evaporation by resistance-heating evaporation and electron-beamevaporation, and in addition, there are methods such as deposition byresistance-heating evaporation and sputtering and deposition byelectron-beam evaporation and sputtering. The first layer 111 can beformed by combining the same type of methods or different types ofmethods. In addition, the example described above shows a layerincluding two kinds of materials. However, when three or more kinds ofmaterials are included, the first layer 111 can be formed also in thesame way by combining the same type of methods or different types ofmethods as described above.

In the light-emitting element described above, the difference betweenthe electron affinity of the electron transporting material included inthe third layer 113 and the electron affinity of the material includedin the layer in contact with the third layer 113 among the layersincluded in the second layer 112 is preferably 2 eV or less, morepreferably 1.5 eV or less. Alternatively, when the third layer 113 iscomposed of an n-type semiconductor, the difference between the workfunction of the n-type semiconductor and the electron affinity of thematerial included in the layer in contact with the third layer 113 amongthe layers included in the second layer 112 is preferably 2 eV or less,more preferably 1.5 eV or less. By joining the second layer 112 and thethird layer 113 as described above, electrons can be injected moreeasily from the third layer 113 to the second layer 112.

It is to be noted that the present invention has a feature that the filmthicknesses of the first to third layers 111 to 113 are determined so asto reduce the reflectivity of the first electrode 101 with respect to anemission wavelength of emitted light, and is not to be consideredlimited to the structure of the light-emitting element shown in FIG. 3.For example, there may be a case where the electron transporting layer124 is not provided although the structure provided with the electrontransporting layer 124 formed in contact with the third layer 113 isshown. Accordingly, the light-emitting layer 123 in contact with thethird layer 113 is provided. In this case, a material for dispersing aluminescent material is preferably used for the light-emitting layer123. Also, it may well be that the electron transporting layer 124 isnot provided.

In addition, a material that is capable of producing luminescencewithout being dispersed, such as Alq₃, can be used for thelight-emitting layer 123. Since the material such as Alq₃ is aluminescent material that has a favorable carrier transporting property,a layer composed of only Alq₃ can function as a light-emitting layerwithout dispersing Alq₃. In this case, the light-emitting layer 123corresponds to a luminescent material itself.

These first to third layers 111 to 113 can be formed by the same method,and can be therefore formed continuously without being exposed to theair. Impurity mixing into an interface and the like can be reduced byforming the first to third layers 111 to 113 continuously without beingexposed to the air in this way.

Next, the electrodes will be described. Each of the first electrode 101and the second electrode 102 are formed by using a conductive material.Further, the second electrode 102 provided on the side from which lightfrom the light-emitting layer is extracted outside needs to have alight-transmitting property in addition to conductivity. Thelight-transmitting property can be obtained also by forming a quite thinfilm composed of a material that has no light-transmitting property.

As a material for the first electrode 101, a metal material that has ahigh reflectivity (reflective material), such as titanium (Ti), aluminum(Al), gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium(Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), or palladium(Pd), can be used. Further, the first electrode 101 can be formed, forexample, by sputtering or evaporation. However, the material for thefirst electrode 101 is not limited to these. Even when the firstelectrode 101 is formed with the use of a metal material that has a highreflectivity in this way, the present invention has a feature ofreducing the reflectivity with respect to an emission wavelength ofemitted light.

In addition, a single layer of the metal material mentioned above or alamination layer can be used for the first electrode 101.

Further, as a material for the second electrode 102, highlylight-transmitting materials such as indium tin oxide (ITO), indium tinoxide containing silicon oxide (ITSO), and indium oxide containing 2 to20% zinc oxide can be used. In addition, it is possible to use a thinfilm that is formed with the use of a metal material such as gold (Au),platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum(Mo), iron (Fe), cobalt (Co), copper (Cu), or palladium (Pd) so as tohave a light-transmitting property. However, the material for the firstelectrode 101 is not limited to these.

In addition, a single layer of the metal material mentioned above or alamination layer can be used for the second electrode 102. When alamination layer is used for the second electrode 102, it is alsopossible to use a structure of forming a thin film of the materialmentioned above and laminating a light-transmitting material thereon. Ofcourse, the second electrode 102 may be formed with the use of the thinmaterial as a single layer. In order to prevent the resistance fromincreasing by forming the second electrode 102 to be thin, an auxiliarywiring can also be provided. Further, the use of a lamination layer canprevent the resistance from increasing.

It is to be noted that the first electrode 101 or the second electrode103 can be an anode or a cathode depending on a voltage that is appliedto the light-emitting element. In the case of an anode, a material thathas a larger work function (a work function of 4.0 eV or more) is used.Alternatively, in the case of a cathode, a material that has a smallerwork function (a work function of 3.8 eV or less) is used.

The first electrode 101 or the second electrode 102 can be formed bysputtering, evaporation, or the like. In the case of using evaporation,the first electrode 101, the first to third layers 111 to 113, and thesecond electrode 102 can be formed continuously without being exposed tothe air. Impurity mixing into an interface and the like can be reducedby forming the light-emitting element continuously without being exposedto the air in this way.

In addition, according to the present invention, when a layer in whichan organic compound and a metal oxide are mixed for a layer that is madethicker, a light-emitting device achieving lower power consumption canbe provided. Further, quenching of luminescence can be prevented sincethe light-emitting layer can be kept further away from the firstelectrode or the second electrode by making the layer thicker.Furthermore, since the light-emitting element can be formed to bethicker, a short circuit between the electrodes can be prevented, andthe mass productivity can be enhanced.

According to the present invention, which provides the thus describedstructure of the light-emitting element, reflection of light from thelight-emitting element at the first electrode 101 or the secondelectrode 102 can be reduced, and a color filter 105 makes it possibleto prevent wavelengths other than a luminescent color from thelight-emitting element from being transmitted. Accordingly, the contrastcan be enhanced by adjusting only light from the light-emitting element,and a light-emitting device that requires no polarization plate or thelike can be provided.

Embodiment Mode 4

In the present embodiment mode, the structure of a light-emittingelement that is different from the embodiment modes described above willbe described.

As shown in FIG. 4, the light-emitting element shown in the presentembodiment mode has a first electrode 101 and a second electrode 102that are opposed to each other, and has a first layer 111, a secondlayer 112, a third layer 113, and a fourth layer 128 that are stacked inthis order from the first electrode 101 side, where it is a feature thatthe fourth layer 128 is provided. The fourth layer 128 can be formed byusing the same material as the first layer 111, and the other structureis the same as the embodiment described above. Therefore, description ofthe structure other than the fourth layer 128 is omitted.

When the fourth layer 128 is provided in this way, damage to therespective layers during forming the second electrode 102 can bereduced.

Further, the film thickness of any one of the first layer 111 to thefourth layer 128 is varied depending on each luminescent color.Accordingly, the reflectivity of the emission wavelength at the firstelectrode 101 can be reduced. In addition, in the case of varying thefilm thickness, a layer in which an organic compound and an inorganiccompound are mixed is preferably used also for the fourth layer 128.Specifically, a vanadium oxide, a molybdenum oxide, a niobium oxide, arhenium oxide, any one of a tungsten oxide, a ruthenium oxide, atitanium oxide, a chromium oxide, a zirconium oxide, a hafnium oxide,and a tantalum oxide can be used as a metal oxide, and nitrides andoxynitrides of these metals may be also used. These metal oxides and thelike, which can be made thicker without increase in driving voltage, arepreferable as described above.

Further reduction in damage during forming the second electrode 102 canbe expected by making the fourth layer 128 thicker.

According to the present invention, which provides the thus describedstructure of the light-emitting element, reflection of light from thelight-emitting element at the first electrode 101 or the secondelectrode 102 can be reduced, and a color filter 105 makes it possibleto prevent wavelengths other than a luminescent color from thelight-emitting element from being transmitted. Accordingly, the contrastcan be enhanced by adjusting only light from the light-emitting element,and a light-emitting device that requires no polarization plate or thelike can be provided.

Embodiment Mode 5

In the present embodiment mode, the cross-sectional structure of a pixelwill be described, where a transistor that controls supply of current toa light-emitting element (referred to as a driving transistor) is ap-channel TFT. It is to be noted that a case where a first electrode anda second electrode respectively serve as an anode and a cathode will bedescried in the present embodiment mode.

FIG. 5 shows a cross-sectional view of three pixels, where a TFT 611 isa p-channel transistor, and light emitted from a light-emitting element603 is extracted from a second electrode 102 side (top emission). InFIG. 5, a first electrode 101 of the light-emitting element 603 and theTFT 611 are electrically connected to each other. Further, anelectroluminescent layer 605 adjacent to the first electrode 101 and asecond electrode 102 adjacent to the electroluminescent layer 605 arestacked in order. The electroluminescent layer 605 includes the first tothird layers 111 to 113 described above, and further includes the fourthlayer 128. From this electroluminescent layer, light that has a peakwavelength in the visible region in an emission spectrum is emitted.

The TFT 611 has a channel forming region formed with the use of aseparated island-shaped semiconductor film that is 10 to 200 nm inthickness. For the semiconductor film, any of an amorphous semiconductorfilm, a crystalline semiconductor film, a microcrystalline semiconductorfilm may be used. For example, in the case of a crystallinesemiconductor film, an amorphous semiconductor film is formed first, anda crystalline semiconductor film crystallized by heat treatment can beused. For the heat treatment, a heating furnace, laser irradiation,irradiation with light emitted from a lump (hereinafter, referred to aslamp anneal) instead of laser light, or a combination thereof can beused.

In the case of using laser irradiation, a continuous-wave laser (CWlaser) or a pulsed-oscillation laser (pulsed laser) can be used.

Further, the laser incident angle may be made to be θ (0°<θ<90°) withrespect to the semiconductor film. The result is that laser interferencecan be prevented.

It is to be noted that laser light of a continuous-wave fundamental waveand laser light of a continuous-wave harmonic may be used for laserirradiation, or laser light of a continuous-wave fundamental wave andlaser light of a pulsed-oscillation harmonic may be used for laserirradiation. Plural laser light irradiation makes it possible to coverenergy.

Alternatively, in the case of using a pulsed-oscillation laser, laserlight is emitted at an oscillation frequency to provide acontinuously-grown crystal grain in a scanning direction, where theoscillation frequency is a frequency at which a semiconductor film canbe irradiated the next pulsed laser light between melting of thesemiconductor film by laser light and solidification thereof. In short,a pulsed beam that is emitted at an oscillation frequency with a lowerlimit can be used in such a way that the period of pulsed oscillation isshorter than the time from melting of a semiconductor film to completesolidification thereof. The oscillation frequency of a pulsed beam thatcan be actually used is 10 MHz or more, and a frequency band much higherthan a usually-used frequency band of several ten to several hundred Hzis used.

As another crystallization method by heat treatment, in the case ofusing a heating furnace, there is a method of heating an amorphoussemiconductor film at 500 to 550° C. for 2 to 20 hours. In this case,the temperature is controlled preferably by multistep regulation in the500 to 550° C. range so as to gradually get higher. Since hydrogen andthe like in the semiconductor film are released in the initial heatingprocess at a lower temperature, film roughness by crystallization can bereduced, and further, a dangling bond can be terminated. Moreover, it ispreferable to provide a metal element that promotes crystallization, forexample, Ni, on the amorphous semiconductor film since the heatingtemperature can be made lower. Even in the case of crystallization usingthis metal element, the semiconductor film may be heated to 600 to 950°C.

However, in the case of providing the metal element, there is fear thatadverse effects are caused on electrical characteristics. Therefore, itis necessary to perform a gettering process for reducing or removing themetal element. For example, a process of capturing the metal elementwith an amorphous semiconductor film as a gettering sink may beperformed.

Further, the TFT 611 has a gate insulating film covering thesemiconductor film and a gate electrode of stacked first and secondconductive films, and an insulating film containing hydrogen is providedon the gate electrode. A dangling bond can be terminated also by thehydrogen.

The TFT 611 is a p-channel transistor, and has a single-drain structurein which the semiconductor film includes only a higher-concentrationimpurity region. Alternatively, the TFT 611 may have an LDD (LightlyDoped Drain) structure in which a semiconductor film includes alower-concentration impurity region and a higher-concentration impurityregion. It is to be noted that the TFT 611 may have a GOLD structure inwhich a lower-concentration impurity region is overlapped with a gateelectrode.

The TFT 611 is covered with an interlayer insulating film 607, and apartition 608 with an opening is formed on the interlayer insulatingfilm 607. A portion of the first electrode 101 is exposed at the openingof the partition 608, and the first electrode 101, theelectroluminescent layer 605, and the second electrode 102 are stackedin order at the opening.

The electroluminescent layer 605 corresponds to the first layer 111, thesecond layer 112, the third layer 113, and additionally the fourth layer128, and the film thickness thereof is determined so that reflection ofa luminescent color from the electroluminescent layer 605 at the firstelectrode 101 is reduced. In addition, when a mixed layer including anorganic compound and a metal oxide is used for these layers, increase indriving voltage due to the film made thicker can be prevented.

Since the top emission type is employed, the first electrode 101 has nolight-transmitting property whereas the second electrode 102 has alight-transmitting property. For these materials, it is possible torefer to the embodiment mode described above.

The electroluminescent layer 605 has a layer that generates holes, alayer that generates electrons, and the like in addition to alight-emitting layer as described above. Since the first electrode 101serves as an anode, a layer that generates holes, a light-emittinglayer, and a layer that generates electrons are in this order stackedfrom the first electrode 101 side. It is to be noted that a layer thatgenerates electrons, a light-emitting layer, and a layer that generatedholes are in this order stacked when the first electrode 101 serves as acathode.

In the case of the pixels shown in FIG. 5, light emitted from thelight-emitting element 603 can be extracted from the second electrode102 side as indicated by an outline arrow. Further, reflection of thelight at the first electrode 101 is reduced. Specifically, the filmthickness of the electroluminescent layer 605 is determined so thatreflection at the first electrode 101 is reduced.

Moreover, the color filter 105 provided on the second electrode sidemakes it possible to prevent wavelengths other than a luminescent colorfrom the light-emitting element from being transmitted, namely, thecolor filter 105 is able to make a wavelength of the luminescent colorfrom the light-emitting element transmit selectively. Accordingly, thecontrast and the like can be controlled by adjusting only light from theelectroluminescent layer, and further, the structure according to thepresent invention can provide a light-emitting device requires nopolarization plate or the like.

Embodiment Mode 6

Next, a cross-sectional view of three pixels will be described withreference to FIG. 6, where a TFT 611 is a p-channel transistor, andlight emitted from a light-emitting element 603 is extracted from afirst electrode 101 side (bottom emission).

In FIG. 6, the first electrode 101 of the light-emitting element 603 andthe TFT 611 are electrically connected to each other. Further, anelectroluminescent layer 605 and a second electrode 102 are stacked inorder over the first electrode 101.

The TFT 611 can be formed in the same way as in the embodiment modedescribed above. In addition, since the bottom emission type isemployed, the first electrode 101 has a light-transmitting propertywhereas the second electrode 102 has no light-transmitting property. Forthese materials, it is possible to refer to the embodiment modedescribed above. Further, a color filter 105 is provided on the firstelectrode 101 side to which light is emitted.

The electroluminescent layer 605 can be formed also in the same way asin the embodiment mode described above. However, a layer that generatedelectrons, a light-emitting layer, and a layer that generates holes arestacked preferably in this order in the present embodiment mode sincethe first electrode 101 serves as a cathode. Further, the filmthicknesses of the first to third layers 111 to 113 and additionally thefourth layer 128 are determined so that reflection of a luminescentcolor from the electroluminescent layer 605 at the second electrode 102can be reduced. Since FIG. 6 shows a case of a bottom-emission type, itis preferable in the electroluminescent layer 605 to determine the filmthickness of a layer corresponding to the third layer 113 that is closerto the second electrode 102.

In the case of the pixels shown in FIG. 6, light emitted from thelight-emitting element 603 can be extracted from the first electrode 101side as indicated by an outline arrow, reflection at the secondelectrode 102 can be reduced, and a color filter 105 makes it possibleto prevent wavelengths other than a luminescent color from thelight-emitting element from being transmitted. Accordingly, the contrastcan be enhanced by adjusting only light from the electroluminescentlayer, and a light-emitting device that requires no polarization plateor the like can be provided.

Embodiment Mode 7

In the present embodiment mode, an equivalent circuit diagram of a pixelincluding a light-emitting element will be described with reference toFIGS. 7A to 7D.

FIG. 7A shows an example of an equivalent circuit for a pixel, whichincludes a signal line 712, a power supply line 715, and a scan line710, and at an intersecting portion thereof, a light-emitting element603, transistors 703 and 711, and a capacitor 704.

In this equivalent circuit, a video signal is input to the signal line712 from a signal line driving circuit. The transistor 711 is able tocontrol supply of the potential of the video signal to a gate of thetransistor 703 in accordance with a selection signal that is input tothe scan line 710, and is referred to as a switching transistor. Thetransistor 703 is able to control supply of current to thelight-emitting element 603 in accordance with the potential of the videosignal, and is referred to as a driving transistor. The light-emittingelement 603 goes into an emitting state or non-emitting state inaccordance with supplied current, which makes it possible to displayimages. The capacitor 704 is able to hold a voltage between the gate andsource of the transistor 703. It is to be noted that, although thecapacitor 704 is shown in FIG. 4A, it is not necessary that thecapacitor 704 be provided when the gate capacitance of the transistor703 or another parasitic capacitance is enough.

FIG. 7B is an equivalent circuit diagram of a pixel where a transistor718 and a scan line 719 are additionally provided to the pixel shown inFIG. 7A.

The transistor 718 makes it possible to make the potentials of the gateand source of the transistor 703 equal to each other so that a state inwhich no current flows into the light-emitting element 603 can beforcibly made, and is referred to as an erasing transistor. Therefore,in time gray scale display, the next image signal can be input beforeinputting image signals into all pixels, and the duty ratio can be thusmade higher.

Alternatively, an element that functions as a diode may be providedinstead of the transistor 718. In the present embodiment mode, adiode-junction transistor or a pn-junction diode can be provided betweenthe gate electrode of the transistor 703 and the scan line 719. Then, astate in which no current flows into the light-emitting element 603 canbe forcibly made.

FIG. 7C is an equivalent circuit diagram of a pixel where a transistor725 and a wiring 726 are additionally provided to the pixel shown inFIG. 7B. The gate of the transistor 725 has a fixed potential. Forexample, the potential of the gate is fixed by connecting the gate tothe wiring 726. In addition, the transistors 703 and 725 are connectedin series between the power supply line 715 and the light-emittingelement 603. Therefore, in FIG. 7C, the transistor 725 is able tocontrol the value of current supplied to the light-emitting element 603whereas the transistor 703 is able to control whether or not the currentis supplied to the light-emitting element 603.

The equivalent circuits of the pixels shown in FIGS. 7A to 7C can bedriven by a digital method. In the case of driving by a digital method,some variations in electrical characteristics of each driving transistorare negligible, if any, since the transistors are used as switchingelements.

An equivalent circuit for a pixel of a light-emitting device accordingto the present invention can be driven by either a digital method or ananalog method. For example, an equivalent circuit for a pixel shown inFIG. 7D includes a signal line 712, a power supply line 715, and a scanline 710, and at an intersecting portion thereof, a light-emittingelement 603, transistors 711, 720, 721, and a capacitor 704. In FIG. 7D,the transistors 720 and 721 constitute a current mirror circuit, whichis composed of p-channel transistors. In this equivalent circuit for apixel, in the case of a digital method, a digital video signal is inputfrom the signal line 712, and the value of current supplied to thelight-emitting element 603 is controlled in accordance with a time grayscale. Alternatively, in the case of an analog method, an analog videosignal is input from the signal line 712, and the value of currentsupplied to the light-emitting element 603 is controlled in accordancewith the value of the analog video signal. In the case of driving by theanalog method, lower power consumption can be achieved.

In each pixel described above, signals are input to the signal line 712,the power supply line 715, and the wiring 726 from a signal line drivingcircuit. In addition, signals are input to the scan lines 710 and 719from a scan line driving circuit. One or more signal line drivingcircuits and one or more scan line driving circuits can be provided. Forexample, first and second scan line driving circuits can be providedwith a pixel portion interposed therebetween.

In addition, in the pixel shown in FIG. 7A, a state in which no currentflows into the light-emitting element 603 can be forcibly made asdescribed with reference to FIG. 7B. For example, the transistor 711 isselected by a first scan line driving circuit at the moment when thelight-emitting element 603 lights up, and a signal for forcibly applyingno current into the light-emitting element 603 is supplied to the scanline 710 by a second scan line driving circuit. The signal for forciblyapplying no current (Write Erase Signal) is a signal for applying apotential so that first and second electrode 101 and 102 of thelight-emitting element have the same potential. In this way, a state inwhich no current flows into the light-emitting element 603 can beforcibly made, and the duty ratio can be thus made higher.

As described above, various types of equivalent circuits can be employedfor a pixel of a light-emitting device according to the presentinvention. It is to be noted that the pixel circuit according to thepresent invention is not limited to the structures described in thepresent embodiment mode, and the present embodiment can be freelycombined with the embodiment modes described above.

Embodiment Mode 8

Electronic devices provided with a light-emitting device according tothe present invention include a television set (also, simply referred toas a TV or a television receiver), a digital camera, a digital videocamera, a cellular phone unit (also, simply referred to as amobile-phone unit or a cellular phone), a mobile information terminalsuch as PDA, a portable game machine, a monitor for a computer, acomputer, a sound reproduction device such as a in-car audio system, animage reproduction device provided with a recording medium such as ahome-use game machine, and the like. Specific examples thereof will bedescribed with reference to FIGS. 8A to 8F.

A mobile terminal device shown in FIG. 8A includes a main body 9201, adisplay portion 9202, and the like. A light-emitting device according tothe present invention can be applied to the display portion 9202.Accordingly, it is possible to provide a mobile terminal device that isable to enhance the contrast thereof and requires no polarization plateor the like.

A digital video camera shown in FIG. 8B includes a display portion 9701,a display portion 9702, and the like. A light-emitting device accordingto the present invention can be applied to the display portion 9701.Accordingly, it is possible to provide a digital video camera that isable to enhance the contrast thereof and requires no polarization plateor the like.

A cellular phone shown in FIG. 8C includes a main body 9101, a displayportion 9102, and the like. A light-emitting device according to thepresent invention can be applied to the display portion 9102.Accordingly, it is possible to provide a cellular phone that is able toenhance the contrast thereof and requires no polarization plate or thelike.

A portable television set shown in FIG. 8D includes a main body 9301, adisplay portion 9302, and the like. A light-emitting device according tothe present invention can be applied to the display portion 9302.Accordingly, it is possible to provide a portable television set that isable to enhance the contrast thereof and requires no polarization plateor the like. In addition, the light-emitting device according to thepresent invention can be applied to various types of portable televisionsets such as a small-sized television incorporated in a mobile terminalsuch as a cellular phone handset and a medium-sized television that isportable.

A portable computer shown in FIG. 8E includes a main body 9401, adisplay portion 9402, and the like. A light-emitting device according tothe present invention can be applied to the display portion 9402.Accordingly, it is possible to provide a portable computer that is ableto enhance the contrast thereof and requires no polarization plate orthe like.

A television set shown in FIG. 8F includes a main body 9501, a displayportion 9502, and the like. A light-emitting device according to thepresent invention can applied to the display portion 9502. Accordingly,it is possible to provide a television set that is able to enhance thecontrast thereof and requires no polarization plate or the like.

As described above, a light-emitting device according to the presentinvention makes it possible to provide a mobile terminal device that isable to enhance the contrast thereof and requires no polarization plateor the like.

Embodiment 1

The present embodiment shows actual measurement values, calculatedvalues, and the like of the reflectivities or transmittances ofwavelengths from 400 to 800 nm.

FIG. 9 shows a result of using a red color filter for providing a redcolor. The element has a structure in which titanium (Ti) as a firstelectrode, a mixed layer (196 nm thick) including organic compounds(α-NPD and rubrene) and a molybdenum oxide, α-NPD 10 nm thick, a layer(40 nm thick) in which a red luminescent material is dispersed in Alq₃,Alq₃ 20 nm thick, a layer (20 nm thick) including BzOS and lithium (Li),and aluminum (Al 15 nm thick) as a second electrode are stacked inorder. It is to be noted that it is enough for the titanium to have afilm thickness through which no light is transmitted, which is 40 nm ormore.

Curve (A) shows calculated values of reflectivity at the firstelectrode, from which it is determined that the reflectivity has thelowest value around 630 nm, which corresponds to an emission spectrumfrom the light-emitting element. Curve (B) shows actual measurementvalues of transmittance of the red color filter (film thickness: 1.5μm). It is determined from Curve (B) that the transmittance has thehighest value around 630 nm, which corresponds to an emission spectrumfrom the light-emitting element. Curve (B) shows values obtained bymultiplying the reflectivity (Curve (A)) by the transmittance (Curve(B)) squared, which are obtained for considering reflection of outsidelight in an actual light-emitting device. This is because outside lightpasses through the color filter, is reflected at the first electrode,passes trough the color filter again, and is recognized by a user. It isdetermined from Curve (C) that the reflectivity is small enough around630 nm, which corresponds to an emission spectrum from thelight-emitting element. Specifically, reflectance of the light emittingelement at the peak wavelength (around 630 nm) is 10% or less.

Further, FIG. 10 shows a result of using a green color filter forproviding a green color. The element using the same materials, which hasa structure in which titanium (Ti) as a first electrode, a mixed layer(140 nm thick) including organic compounds (α-NPD and rubrene) and amolybdenum oxide, α-NPD 10 nm thick, a layer (40 nm thick) in which agreen luminescent material is dispersed in Alq₃, Alq₃ 20 nm thick, alayer (20 nm thick) including BzOS and lithium (Li), and aluminum (Al 15nm thick) as a second electrode are stacked in order, is different fromthe red element described above in the film thickness of the mixed layerincluding the organic compounds (α-NPD and rubrene) and the molybdenumoxide. It is to be noted that it is enough for the titanium to have afilm thickness through which no light is transmitted, which is 40 nm ormore.

Curve (A) shows calculated values of reflectivity at the firstelectrode. It is determined that the reflectivity has the lowest valuearound 530 nm, which corresponds to an emission spectrum from thelight-emitting element. Curve (B) shows actual measurement values oftransmittance of the green color filter (film thickness: 1.5 μm). It isdetermined from Curve (B) that the transmittance has the highest valuearound 530 nm, which corresponds to an emission spectrum from thelight-emitting element. Curve (B) shows values obtained by multiplyingthe reflectivity (Curve (A)) by the transmittance (Curve (B)) squared.It is determined from Curve (C) that the reflectivity is small enougharound 530 nm, which corresponds to an emission spectrum from thelight-emitting element. Specifically, reflectance of the light emittingelement at the peak wavelength (around 530 nm) is 10% or less.

Further, FIG. 11 shows a result of using a blue color filter forproviding a blue color. The element using the same materials, which hasa structure in which titanium (Ti) as a first electrode, a mixed layer(95 nm thick) including organic compounds (α-NPD and rubrene) and amolybdenum oxide, α-NPD 10 nm thick, t-BuDNA 40 nm thick, Alq₃ 20 nmthick, a layer (20 nm thick) including BzOS and lithium (Li), andaluminum (Al 15 nm thick) as a second electrode are stacked in order, isdifferent from the red and green elements described above in the filmthickness of the mixed layer including the organic compounds (α-NPD andrubrene) and the molybdenum oxide. It is to be noted that it is enoughfor the titanium to have a film thickness through which no light istransmitted, which is 40 nm or more.

Curve (A) shows calculated values of reflectivity at the firstelectrode. It is determined that the reflectivity has the lowest valuearound 450 nm, which corresponds to an emission spectrum from thelight-emitting element. Curve (B) shows actual measurement values oftransmittance of the blue color filter (film thickness: 1.5 μm). It isdetermined from Curve (B) that the transmittance has the highest valuearound 450 nm, which corresponds to an emission spectrum from thelight-emitting element. Curve (B) shows values obtained by multiplyingthe reflectivity (Curve (A)) by the transmittance (Curve (B)) squared.It is determined from Curve (C) that the reflectivity is small enougharound 450 nm, which corresponds to an emission spectrum from thelight-emitting element. Specifically, reflectance of the light emittingelement at the peak wavelength (around 450 nm) is 10% or less.

This light-emitting element makes it possible to reduce reflection of adesired emission wavelength at the first electrode, and the colorfilter, which has a high transmittance to the emission wavelength fromthe light-emitting element, is able to absorb wavelengths of the otherlight. Accordingly, it is possible to consider only light from thelight-emitting element, and contrast control can be thus made simpler.Further, according to the present invention, a top-emissionlight-emitting device that requires no polarization plate or the likecan be provided.

Although the reflectivity of the first electrode is considered in thepresent embodiment, the reflectivity of the second electrode can be alsoconsidered in the same way. Contrast control can be made simpler byconsidering the reflectivity of the second electrode and determining thefilm thickness of the light-emitting element, and a bottom-emissionlight-emitting device that requires no polarization plate or the likecan be thus provided.

Based on this simulation result, the film thickness of anelectroluminescent layer can be determined with respect to a first orsecond electrode. In fact, a light-emitting device can be manufacturedby measuring the reflectivity of an emission wavelength from alight-emitting element at a first or second electrode, measuring thetransmittance of a desired color filter, and increasing the transmissionof the emission wavelength from the light-emitting element.

Although the present invention has been fully described by way ofexample with reference to the accompanying drawings, it is to beunderstood that various changes and modifications will be apparent tothose skilled in the art. Therefore, unless such changes andmodifications depart from the scope of the present invention hereinafterdefined, they should be construed as being included therein.

1-28. (canceled)
 29. A method for manufacturing a light-emitting devicecomprising, wherein the light-emitting device comprises: a firstelectrode; a second electrode provided to be opposed to the firstelectrode; an electroluminescent layer provided between the firstelectrode and the second electrode; a light-emitting element comprisingthe first electrode, the second electrode, and the electroluminescentlayer; and a color filter through which light from the light-emittingelement is transmitted, wherein any one layer of the electroluminescentlayer is formed to have a thickness that reduces reflection of anemission wavelength from the light-emitting element at the firstelectrode, and wherein the color filter is formed to have a hightransmittance with respect to the emission wavelength from thelight-emitting element.
 30. A method for manufacturing a light-emittingdevice comprising, wherein the light-emitting device comprises: a firstelectrode; a second electrode provided to be opposed to the firstelectrode; an electroluminescent layer provided between the firstelectrode and the second electrode; a light-emitting element comprisingthe first electrode, the second electrode, and the electroluminescentlayer; and a color filter through which light from the light-emittingelement is transmitted, wherein the light-emitting element emitsmonochromatic light, wherein any one layer of the electroluminescentlayer is formed to have a thickness that reduces reflection of anemission wavelength of a luminescent color from the monochromaticlight-emitting element at the first electrode, and wherein the colorfilter is formed to have a high transmittance with respect to theemission wavelength from the monochromatic light-emitting element.
 31. Amethod for manufacturing a light-emitting device comprising, forming afirst electrode; forming a light emitting layer over the firstelectrode; forming a second electrode over the light emitting layer;forming a color filter at the second electrode side, and wherein areflectance of light emitted from said light emitting layer at the firstelectrode is 10% or less.
 32. A method for manufacturing alight-emitting device comprising, forming a first electrode; forming alight emitting layer over the first electrode; forming a secondelectrode over the light emitting layer; forming a color filter at thesecond electrode side, and wherein a reflectance of light emitted fromsaid light emitting layer at the first electrode is 10% or less, andwherein visible light having at least peak wavelength passes through thecolor filter.
 33. A method for manufacturing a light-emitting devicecomprising, forming a first electrode; forming a light emitting layerover the first electrode; forming a second electrode over the lightemitting layer; forming a color filter at the second electrode side, andwherein a reflectance of light emitted from said light emitting layer atthe first electrode is 10% or less, wherein visible light having atleast peak wavelength passes through the color filter, and wherein thelight emitting layer emits monochromatic light.
 34. A method formanufacturing a light-emitting device comprising, forming a firstelectrode; forming a electroluminescent layer having a light emittinglayer and a layer including a metal oxide over the first electrode;forming a second electrode over the light emitting layer; forming acolor filter at the second electrode side, and wherein the layerincluding a metal oxide is formed so that a reflectance of light emittedfrom said light emitting layer at the first electrode is 10% or less,wherein visible light having at least peak wavelength passes through thecolor filter, and wherein the light emitting layer emits monochromaticlight.
 35. The method according to claim 29, wherein the layer of theelectroluminescent layer is formed by using a layer including a metaloxide selected from the group consisting of a vanadium oxide, amolybdenum oxide, a niobium oxide, a rhenium oxide, a tungsten oxide, aruthenium oxide, a titanium oxide, a chromium oxide, a zirconium oxide,a hafnium oxide, and a tantalum oxide.
 36. The method according to claim30, wherein the layer of the electroluminescent layer is formed by usinga layer including a metal oxide selected from the group consisting of avanadium oxide, a molybdenum oxide, a niobium oxide, a rhenium oxide, atungsten oxide, a ruthenium oxide, a titanium oxide, a chromium oxide, azirconium oxide, a hafnium oxide, and a tantalum oxide.
 37. The methodaccording to claim 34, wherein the layer of the electroluminescent layeris formed by using a layer including a metal oxide selected from thegroup consisting of a vanadium oxide, a molybdenum oxide, a niobiumoxide, a rhenium oxide, a tungsten oxide, a ruthenium oxide, a titaniumoxide, a chromium oxide, a zirconium oxide, a hafnium oxide, and atantalum oxide.
 38. The method according to claim 29, wherein the firstelectrode is formed by using a reflective material.
 39. The methodaccording to claim 30, wherein the first electrode is formed by using areflective material.
 40. The method according to claim 31, wherein thefirst electrode is formed by using a reflective material.
 41. The methodaccording to claim 32, wherein the first electrode is formed by using areflective material.
 42. The method according to claim 33, wherein thefirst electrode is formed by using a reflective material.
 43. The methodaccording to claim 34, wherein the first electrode is formed by using areflective material.
 44. The method according to claim 29, wherein thesecond electrode is formed by using a light-transmitting material. 45.The method according to claim 30, wherein the second electrode is formedby using a light-transmitting material.
 46. The method according toclaim 31, wherein the second electrode is formed by using alight-transmitting material.
 47. The method according to claim 32,wherein the second electrode is formed by using a light-transmittingmaterial.
 48. The method according to claim 33, wherein the secondelectrode is formed by using a light-transmitting material.
 49. Themethod according to claim 34, wherein the second electrode is formed byusing a light-transmitting material.